The present invention relates to a method of operating a fossil fuel-fired furnace and, more particularly, to a method of operating a pulverized coal-fired furnace so as to control the flow of combustion products therein. The present invention also relates to a fossil fuel-fired furnace such as a pulverized coal-fired furnace.
U.S. Pat. No. 4,672,900 to Santalla et al. discloses an arrangement for a tangentially-fired, pulverized coal-burning furnace in which one or more nozzles are mounted in the upper portion of the combustion chamber of the furnace to eject secondary air in opposition to the swirling fireball flowing in the upper portion of the combustion chamber. The secondary air ejected by the nozzle or nozzles in the upper portion of the combustion chamber is ejected in a manner such that the secondary air provides equal but opposite angular momentum to the angular momentum of the fuel and air introduced into the lower portion of the combustion chamber. Such an arrangement, according to the patent, results in the elimination of a rotating pattern of the products of combustion which reduces the probability of ash particles migrating to the boundary walls (slagging) while simultaneously providing conditions ideal for flowing into the convection section of the furnace.
It would be desirable to obtain the benefits of the arrangement disclosed in the Santalla et al. patent in other furnace configurations such as a tangential firing furnace configuration in which there is either no separated overfire air compartment or the separated overfire air compartment is not operated to influence the swirling fireball in the same manner as the separated overfire air compartment of the Santalla et al. patent. In such other furnace configurations, the aerodynamic behavior of the swirling fireball as well as other conditions in the furnace may complicate a direct application of an equal and opposite injection of secondary air from a separated overfire air compartment. For example, a mere reconfiguration of several air nozzles in the lower region of the furnace to inject air in an oppositional manner to the swirling fireball may merely result in a change in rotation of the swirling fireball, thus failing to reap the benefits presumably associated with an elimination of a rotating pattern of combustion.
Moreover, there are costs associated with an effort to achieve complete suppression of the rotation of the swirling fireball. Interventions such as injecting additional volumes of air in opposition to the swirling fireball, adjustment of the tilt orientation of the injected air or reduction of the load to completely suppress the formation of any non-uniform (rotational) flow of the flue gas from the fireball engender greater operating expense or less efficiency. Also, the materials and construction of the portion of the furnace which handle the non-uniform flue gas flow such as the convective pass must necessarily be constrained to those materials and construction which can withstand the maximum or peak temperature which may be experienced due to an unmodulated non-uniform flow of flue gas in the convective pass. Thus, the industry would benefit from a method to modulate or control the non-uniform flow of flue gas into the convective pass of a furnace, thereby mitigating or eliminating the undesirable effects of non-uniform flow such as, for example, a maldistribution of energy absorption by the convective heat exchange surface within the convective pass as a result of the differences in the local heat transfer coefficients. Additionally, the industry would benefit from an approach to configuring the tangential firing operation of a pulverized coal-fired furnace that would fully optimize the combustion process benefits associated with control of the swirling fireball created in a tangential firing process.